In the field of the high-power ultra-short laser pulse amplification, the enhancement of the output of a laser system is hampered by a series of factors such as the gain narrowing effect and the gain saturation effect in the solid-state laser amplification medium. The gain narrowing effect is caused by the limited bandwidth of the atomic emission in the laser medium and the frequency pulling effect, which makes the spectrum of the output pulse narrowing that not only leads to the deformity of the pulse waveform but also conducts the chirped pulse hard to be compressed into the original width. The gain saturation effect is another main factor leading to the distortion of CPA. It is caused by the pulse head depletion more the population inversion in the amplification medium than the pulse tail, which leads to the intensity of the pulse head getting more amplification than the intensity of the pulse tail. Hence the output of amplified pulse becomes asymmetric in the pulse waveform, which leads to the lose of amplification efficiency of chirped pulse, which leads to the red shift effect in the spectrum of the laser pulse, and which leads to the distortion of CPA. In order to compensate the gain narrowing and gain saturation effects of CPA, and also to improve the signal to noise ratio (SNR) of the output high-power laser pulse, people needs a kind of spectrum modulation method to shape the spectrum distribution of pre-amplification pulse, making it depression in the vicinity of center wavelength in some shapes and in some depths practically. In the inertial confinement fusion (ICF) field, the physics experiments have harsh requirements on the pulse parameter of the output laser. And lots of attention have been paid to the spatial distribution, the temporal profile, as well as the spectrum shaping in the design of kilojoules peta-watt-class laser system, in order to compensate the gain narrowing and gain saturation effects of CPA, and also to improve the SNR of the output high-power laser pulse, such as in the literatures: [X L Chu, B Zhang, B W Cai, et. al., “Study of the Multipass Amplification of the Chirped Pulse and Its Inverse Problem”, Acta Phys. Sin., 2005, 54(10), 4696-1701], [B Zhang, B D Lü, “An Inverse Problem of Multistage and Multipass Pulsed Laser Amplifiers”, Chin. J. Lasers, 1997, 24(6), 495-500], [T Wang, D Y Fan, “Pulse Shaping Design of High Power Laser Amplifier”, High Power Laser and Particle Beams, 1999, 11(2), 139-142], and [X J Huang, X F Wei, H S Peng et. al. “On Structure of a Hundred Terawatt Class Ultra-short Laser System”, 2004, the conference on optoelectronics in Sichuan].
The above literatures also illustrate some defects in the traditional shaping methods, just as following: 1. Using the thin etalon in regenerative amplification to tune the spectrum of the laser pulse, but it is complexity of the device regulation. In addition, an optical parametric chirped pulse amplifier (OPCPA) has substituted the regenerative amplifier in the front-end of kilojoules peta-watt-class laser system. So this method does not apply. 2. Using the center wavelength blue shift injection, that is, making the centre of the pulse wavelength blue shift relative to the centre wavelength of the medium spontaneous emission, making the long wavelength parts of the pulse with a large duration, and making the spectrum modulation directly, but it is not practical to a over terawatt (1012) system. 3. Using the long-wavelength injection method that utilizes narrowing effect to compensate saturation effect, but this will make the system unstable. 4. Using the acousto-optic programmable dispersive filter (AOPDF) to compensate the spectral gain narrowing, but it has lower sufferance of the high power laser damages. Although it is equipped with a strong capability of gain compensation and dispersion compensation, it gets success just on the structure of a hundred terawatt class ultra-short laser system but not for the Nd:glass system.
At the same time, all the transmitted elements such as the optical filter and the birefringent crystal, and all the organic selection elements such as the hinged organic polymer used for modulating the spectrum of the laser pulse, are unusable for the high-power and huge-energy laser, such as in the literatures [P F Zhu, J X Yang, S L Xue, M R Li, Z Q Lin, “Spectrum Shaping of the Ultrashort Pulse”, CHINESE J. OF LASERS, 2003, 30(12), 1075-1078]; [Y X Leng, L H Lin, Z Z Xu, “Spectrum Shaping in a Ti:Sapphire Regenerative Amplifier”, ACTA OPT. SINICA, 2002, 22(02), 170-174]; the CN patent Ser. No.ZL200410025678.1 and the CN patent Ser. No. ZL200410024684.5.
Furthermore, the above literatures also illustrate some defects in the traditional shaping methods at the present, just as following: 1. Using the interference filter for selective transmission, but it exists difficult both in the principle of design and in the fabrication techniques, such as the principle of design can not avoid the Fabry-Perot (FP) side effect. When the modulation bandwidth is less than tens nanometers, the fabrication techniques can not control the bandwidth and the depth in the centre wavelength of the transmittance curve according to the limit of the coating techniques. At the same time, the incident angle of laser is required at high precision and less flexibility. 2. Using the combination lens of birefringent crystal for the selective transmission, here the interference between the ordinary light and the extraordinary light conducts the selective transmission, but it exists difficulty both in the principle of design and in the fabrication techniques. For example, the transmitted element can not avoid the side effect such as FP effect and the distortion of phase etc.; furthermore, the thickness of the birefringent crystal is determined by the spectrum bandwidth of the original pulse, which the law is: the less the bandwidth is, the more the thickness of the birefringent crystal required is. For instance, if the spectrum bandwidth of the ultra-short pulse is 13 nm, the thickness of the birefringent crystal must be 25 mm; however, the size of birefringent crystal can not be produced that large easily. 3. Using the hinged organic polymer waveguide or using the hinged organic polymer waveguide after a corona poling process for obtaining the characteristics of photoelectric effect for the selective reflection, but it has lower sufferance of the high power laser damages.